Methods of manufacturing optical elements such as lenses by precision mold pressing that afford adequate optical performance without requiring mechanical processing such as grinding or polishing of the optically functional surfaces formed by press molding are known. Further, the management of optical characteristics is indispensable in the manufacturing of optical glass elements. These are determined by the specifications of the optical product in which the optical glass element is employed. Normally, prescribed optical constants (often typically denoted by the refractive index nd and Abbé number v(nu)d) are managed along with their permissible ranges (tolerances). It is known that optical glasses having various ranges of optical constants can be obtained by selecting various glass compositions.
The need for optical materials with high refractive indexes has increased as small image pickup apparatuses such as cameras, videos, and portable terminals have been packed with greater numbers of pixels and rendered more lightweight and compact. Conventionally, glasses with high refractive indexes have been mainly low dispersion glasses. However, there is a need for glasses with high refractive indexes but lower dispersion in small image pickup apparatuses such as those mentioned above. Further, glass materials such as the above that are suited to precision mold pressing are needed to manufacture aspherical lenses with high productivity.
Glass materials for precision molding that have a low Tg are desirable from the perspective of the durability of the pressing mold and mold release film employed. However, a high Tg is unavoidable in satisfying the required optical constants and achieving glass stability.
The present inventors have previously proposed glass materials for precision molding that have high refractive indexes and are in the medium dispersion range. For example, these glass materials have a refractive index nd of 1.70 to 1.90 and an Abbé number v(nu)d of 25 to 65. Since the compositions thereof are selected to satisfy the above optical constants and avoid a tendency to crystallize, these glasses are primarily optical glasses with a Tg of 550° C. or higher (see Japanese Unexamined Patent Publication (KOKAI) No. 2003-267748 and the English family member thereof, U.S. Patent Application 2003211929AA; Japanese Unexamined Patent Publication (KOKAI) Nos. 2002-012443 and 2004-099428 and the English language family members thereof, U.S. Patent Application 2004106507AA; Japanese Unexamined Patent Publication (KOKAI) No. 2005-247613 and the English language family member thereof, U.S. Patent Application 2005197243AA, the entire contents of which are hereby incorporated herein by reference).
The temperature of the heat treatment increases when subjecting an optical element comprised of a high-temperature glass material with a Tg of 550° C. or higher to a heat treatment to achieve a desired refractive index. Thus, not only does the time required for heating and cooling increase, compromising productivity, but in an optical element that is exposed to an elevated temperature for an extended period, there are problems in that components volatize from the surface and volatizing impurities in the furnace adsorb, forming an altered layer on the surface. Conversely, when the heat treatment is conducted at an excessively low temperature, there is inadequate adjustment of the refractive index.
Specifically, when an optical glass comprised of such a high-temperature glass material is preformed into a prescribed volume and shape to form a glass material, when the glass material is molded into a lens by precision mold pressing, and when annealing is then conducted to reduce the residual strain within the lens following mold release, cases of defective external appearance such as clouding and whitening have been observed in the lens surface. Annealing after mold release is conducted in order for removing strain and/or adjusting the refractive index, and is conducted in the vicinity of the Tg or within a suitable temperature range below the Tg. In a glass material in which the Tg is relatively high, the annealing temperature is also set high; for example, from (Tg−50° C.) to (Tg+20° C.). The temperature range is, for example, in the vicinity of 500 to 600° C.
Accordingly, the present invention has for its object to provide a method for manufacturing high-optical-performance mold-pressed lenses in which the defective external appearance of optical elements comprised of high-temperature glass materials with a Tg of 550° C. or higher is prevented.